The success of a Multiple Input Multiple Output (MIMO) system depends to a large extent on the characteristics of the channel. In a Frequency Division Duplex (FDD) system the Uplink (UL) carrier frequency differs from the Downlink (DL) carrier frequency.
In general, an FDD system does not have the same channel in UL as in DL, i.e. it is not reciprocal. This implies that the transmitter must inform the receiver what channel it sees and vice versa, to be able to direct energy in correct directions. Transmitting the full channel matrix is costly, and therefore not desired. An alternative, used in the 3rd Generation Partnership Project (3GPP) standard 36.x is to transmit an index corresponding to a set of weights.
A base station antenna can select one out of a certain number of precoding matrices. These precoding matrices will transmit the streams in predefined directions. However, the freedom to choose precoding weights is rather limited. The current standard does only support a rather course feedback to select precoding.
In other words, there is only a limited set of precoding matrices available, where each precoding matrix relates to certain weights for the antenna, and thus each precoding matrix is related to a certain antenna radiation lobe configuration. If a user terminal supplies feedback to the base station, the base station chooses a certain precoding matrix based on that feedback. That certain precoding matrix provides a certain antenna radiation lobe configuration, which probably is the best antenna radiation lobe configuration among the ones available due to the limited number of precoding matrices available. Even if it is the best antenna radiation lobe configuration, it does probably not provide an ultimate communication situation between the base station and the user terminal. In a worst case, the user terminal may be positioned between two adjacent lobes in the antenna radiation lobe configuration chosen.
It is thus desirable to acquire a more reliable method for providing an acceptable communication situation, where the antenna coverage available is better used.